The USDA ADT (Animal Disease Traceability) has established guidelines for Traceability for Livestock Moved Interstate. These are general guidelines for shows & fairs as well. A Premises ID Number (issued by your state's Dept of Ag/Animal Health) is required for purchase of USDA tags.
Current interstate ID requirements for:
Cattle and Bison - Cattle and bison of any age used for rodeo or recreational events, shows or exhibitions.
Sheep and Goats -Sheep or goats must be identified by an official means of identification, and must remain so identified until they reach their final destination. Options include: Official eartags, including tags approved for use in the SFCP or APHIS-approved premises identification number eartags when combined with a unique animal identification number, USDA backtags or official premises identification backtags that include a unique animal identification number, when used on sheep or goats moving directly to slaughter and when applied within 3 inches of the poll on the dorsal surface of the head or neck, Premises identification eartags or tattoos, if the premises identification method includes a unique animal number or is combined with a flock eartag that has a unique animal number and the animal is accompanied by an owner statement.
Swine - Swine that are required to be officially identified for interstate movement must be identified by an official method. Options include: Official eartags for any swine, USDA backtags, for swine moving to slaughter, For slaughter swine and feeder swine, an eartag or tattoo bearing the premises identification number assigned by the State animal health official to the premises on which the swine originated.
As of 12/1/16, all Oklahoma swine must be tagged with an official form of electronic national identification, a USDA 840 EID tag, before leaving the farm. All Oklahoma show swine, barrows, gilts etc are required to have an 840 EID tag.
In most cases, the health regulations of your state usually are the same for a fair. Many shows choose to exceed the state minimums and standardize the ID used for livestock at the fair. While a variety of Official IDs are currently being accepted, many fairs are moving to a USDA (840) EID tag. A major benefit to going this route (rather than meeting just the state's minimum ID requirements), is that fairs are exposed to significant liability because of being a sanctioning body of co-mingled animals from various locations - differing states and countries.
Regulations can (and do) change, so please double check with your state's animal health commission/department to learn of current specific requirements for your state and situation.
Official Ear tags requirements: